Regulation of complex behavioural transitions: migration to breeding

Abstract

The evolution of migration combines the selective advantages of breeding with ephemeral resources at high latitude or altitude with the benefits of overwintering at distant sites with adequate resources to promote survival. Migration between sites involves changes in behaviour, especially the transition from migration to nesting, or in autumn, the transition to wintering stage. We focus here on the vernal transition. Because migration and onset of nesting are mutually exclusive events, transition from a social, hyperphagic and highly mobile state to territoriality and pair bonding involves complex behavioural changes. In Arctic regions, weather in spring is unpredictable adding another dimension of complexity to the migration/breeding transition. Nevertheless, the brief Arctic summer requires that songbirds arrive on the breeding grounds while conditions may still be unfavourable, thus delaying nesting until conditions ameliorate. This requires flexibility and coordination of migration behaviour to be able to leave if weather deteriorates, or to become territorial, attract a mate and begin nesting within hours when conditions permit. In Gambel's white-crowned sparrows, Zonotrichia leucophrys gambelii, neuroendocrine regulation of reproductive development progresses throughout vernal migration so that males and females are mature when they arrive on the nesting grounds, but final ovarian maturation leading to ovulation and reproductive behaviours is suppressed. What provides the ‘brake’ to the onset of nesting? We focus on the adrenocortical response to acute stress that is greatly increased at arrival, especially in males, probably to enable flexibility in behavioural responses to severe weather. More recent evidence suggests that a hypothalamic peptide, gonadotropin-inhibiting hormone (GnIH) may provide the brake for the onset of nesting without suppressing development of a functional reproductive system. Experimental investigations show that gene silencing of GnIH in the brain increases singing and reproductive behaviours accompanying the onset of nesting. Thus we suggest that corticosteroids and GnIH play important regulatory roles specifically at the transition from vernal migration to the onset of nesting.